But ultimately, I do want to make it so they can't let go. How to do this is more obvious within a more compressive connection based interaction: even if they let go, I am pressing in, so the connection stays (to a large degree). Just like the open-palm exercise. But realizing that this could happen in reverse, within an interaction that more favors the tensile connection has been big in my mind recently. (As they try to let go, force would continue to be exerted on their fingers against my wrist for example)

And all that is in addition to the 2 classic aikido ideas about why uke can't or shouldn't let go: 1. He will be falling if he does, or 2. He will be struck or pushed by the point of contact if he does.

I am trying to keep the 2 types of connection fused into a single percept from my point of view, so that the "inescapable connection" would operate seamlessly in either tensile or compressive directions. After all, I am trying to maintain both of these connections at all times.
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The very few times I have felt that I was fully controlled as uke, neither of the "classic" ideas really applied. I wasn't falling, nor was I in immediate threat of being struck. It was more like the feeling of my center being led around by my arm directly with my brain seemingly having little say in the matter.